The introduction of highly nonlinear fibers (HNLF) has revolutionized optical parametric processing, allowing for the first time the construction of true continuous-wave (CW), high efficiency devices. In contrast to centimeter-long crystalline devices, phase-matched interaction in HNLF can take place over hundreds of meters and have efficiencies exceeding 40 dB. Similar to conventional fiber, microscopic variation in HNLF transverse geometry can lead to spatially-localized dispersion fluctuations. As a result, the phase matching condition varies along the length of the fiber. This variation has been identified as the fundamental obstacle to the construction of wideband parametric devices.
Recent fabrication advances have reduced transverse HNLF variations to multiples of the SiO2 molecular diameter. The ratio between transverse HNLF dimension fluctuation and its longitudinal scale reaches the value of 10−12, making the HNLF one of the most precisely fabricated structures in modern engineering. Indeed, to match this ratio, a typical 100 micron-long monolithic waveguide would have transverse tolerance smaller than the diameter of a single atomic nucleus. The remarkable control of the HNLF cross-sectional variances makes further tightening of the fabrication process a difficult proposition, suggesting a need for alternative means for phase matching control over long length of fiber.
Rather than insisting on unattainable fabrication tolerances (at least according to current state of the art methods), wideband parametric synthesis can be achieved by obtaining nearly exact map of HNLF dispersion fluctuation, which can subsequently be used to either select specific waveguide sections or concatenate fiber segments. Unfortunately, the magnitude of these fluctuations is well below the sensitivity and spatial resolution of existing dispersion measurement techniques. As a result, present research relies on the assumption that the phase matching condition changes randomly along the length of the fiber. In fact, two HNLF sections obtained from a single fiber draw can behave quite differently depending on their local dispersion properties, in spite of their identical global characteristics. Conventional techniques are characterized by sub-km spatial resolution and are designed to analyze fiber types such as conventional single mode fiber (SMF) or non-zero dispersion shifted fiber (NZDSF) that possess high chromatic dispersion. In contrast, wideband parametric synthesis requires meter-scale spatial resolution in nearly dispersionless fibers such as HNLF. In fact, typical HNLF characteristics include nearly 100 times lower dispersion that that of conventional SMF at 1550 nm.
FIGS. 1a and 1b illustrate the basic principle of a commonly used method for dispersion measurement. Co-propagating waves (signal (S) and probe (P)) are launched to generate FPM tones along a specified fiber section. In the case of a highly dispersive fiber, as shown in FIG. 1a, incident pulses can be precisely synchronized in order to generate FPM products along any fiber subsection. Co-propagating pulses separated by Δλ will generate FPM light only within the fiber section in which they are spatially overlapped. The length of this section is defined by the walk-off length:
                                          L            W                    =                                    τ                              Δ                ⁡                                  (                                      1                    /                                          v                      g                                                        )                                                      =                          τ                              D                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                λ                                                    ,                            (        1        )            where D is the fiber dispersion, τ is the pulse duration, and νg is the mean group velocity. As a result, the effective spatial resolution is inversely proportional to the fiber dispersion, allowing for high-resolution measurements in conventional SMF and, to a lesser degree, in NZDSF. In practice, spatial resolution of approximately 100 m is routinely demonstrated in the conventional fiber types.
In the case of nearly dispersionless fibers, such as HNLF, the walk-off length necessarily diverges (LW˜L) and the technique fails to provide spatially resolved measurements. This deficiency is easily visualized with reference to FIG. 1b, by noting that, for nearly equal group velocities, two co-propagating pulses will never separate in the case when launched synchronously. Conversely, if their launch is delayed, the pulses will never collide within the practical length of the fiber under test. While FPM generation will be maximized in the first case, it can only be used to obtain a global, rather than a spatially-localized dispersion measurement. Assuming that two 100 ps-long interacting pulses are separated by 40 nm and launched into HNLF with dispersion slope Sλ=0.025 ps/nm2/km, the corresponding walk-off distance is 5 km. Such a length scale is considerably longer than a typical parametric device length (˜100 m), thus imposing a key limitation to the practical value of this technique.
Accordingly, there remains a need for a new method for measuring dispersion with sufficient sensitivity and spatial resolution to measure the dispersion of HNLF and similar fibers with the ultimate goal of reproducible manufacturing of high quality fiber, making it a viable bulk product line. The present invention directed to such a method.